This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
Of the ten elements of plant food that are absolutely essential, all of them, except carbon and a small quantity of nitrogen, are absorbed by the roots. They are oxygen (the free oxygen of the air is used only in breathing), hydrogen, nitrogen, sulphur, phosphorus, potash, calcium, magnesium, and iron. They are not absorbed in this simple form, but in various combinations termed salts (such as nitrates), acids, etc. Oxygen and hydrogen are absorbed in the form of water; nitrogen in the form of ammonia and nitrates; sulphur as sulphates; phosphorus as phosphates; potash and lime in combination with sulphur, phosphorus, nitrates, &c; and iron in a variety of compounds. Most of the above are present in sufficient quantity in soils generally, and when land requires manuring, nitrogen, phosphorus, and potash are usually most deficient. Lime is occasionally deficient, and is useful for a variety of purposes. Except in the case of Leguminous crops, such as Peas, Broad Beans, Dwarf Beans, and Scarlet Runners, nitrogen is always necessary unless the soil is very fertile. Leguminous plants have bacteria in small nodules upon their roots, and these bacteria are capable of fixing the free nitrogen of the air. Farmyard manures are very valuable in light soils by increasing their power of holding water, independently of the plant food they contain.